1. Field of the Invention
The present invention relates to an abnormality detection apparatus for a vehicle AC generator to be mounted in a passenger car or a truck.
2. Description of the Related Art
Electric power for an automotive vehicle increases year by year because various electric apparatus have been developed and mounted in the automotive vehicle to meet social demand. In order to fulfill the demand for increase in the electric power, the vehicle AC generator is improving while the size thereof is decreasing. In general, the magnitude of damage caused by the abnormal operation thereof increases as the output power of the vehicle AC generator increases. Therefore, it is necessary to alarm a driver of such abnormality of the vehicle AC generator as soon as possible.
A vehicle AC generator includes a field coil for polarizing a pole core that supplies a rotating magnetic field. The output voltage thereof is controlled by controlling conduction rate of a power transistor that supplies field current to the field coil. Such abnormality detection has been known well for a long time. For example, an abnormality is judged if a relationship between the voltage level of a vehicle battery and conduction rate of the field coil is off a predetermined normal value. However, such an abnormality detection apparatus is not only reliable but can not detect abnormality until the vehicle AC generator has completely failed.
JP-A-8-65914, which is entitled “CONTROL APPARATUS OF VEHICLE AC GENERATOR”, discloses another abnormality detection apparatus, and U.S. Pat. No. 4,242,674, which is entitled “ALTERNATOR FAILURE WARNING INDICATOR”, discloses another abnormality detection apparatus. The control apparatus that is disclosed in JP-A-8-65914 judges abnormality by duty ratio of the output wave induced in an armature winding of a vehicle AC generator. On the other hand, the failure warning indicator that is disclosed in U.S. Pat. No. 4,242,674 detects a peak value of ripples of DC output voltage and judges abnormality if there is a ripple whose peak value is equal to or higher than a predetermined value.
The control apparatus disclosed in JP-A-8-65914 can detect abnormality by the duty ratio of the output voltage of the armature winding which changes if there is abnormality. However, if there is abnormality, such as a short circuit in a phase-winding of the armature winding or between two phase-windings, the control apparatus can not detect abnormality because the duty ratio of the output voltage of the armature winding does not change.
Incidentally, wave shapes of an armature winding of a vehicle AC generator when abnormality occurs at various portions of the vehicle AC generator are shown in FIGS. 8A-8E: FIG. 8A shows a normal wave shape; FIG. 8B shows a wave shape when a positive side diode short-circuits; FIG. 8C shows a wave shape when a negative side diode short-circuits; FIG. 8D shows a wave shape when there is a short circuit between two phase-windings; and FIG. 8E shows a wave shape when there is a breaking of wire in the armature winding.
If any of the diodes fails, the duty ratio of the output voltage of the armature winding changes (for example, the duty ratio of 50% at the normal condition changes to 67% if a positive diode short-circuits and to 33% if a negative diode short-circuits). On the other hand, if the armature winding fails, the duty ratio of the output voltage of the armature winding is still 50%. Therefore, the control apparatus disclosed in JP-A-8-65914 can not detect the abnormality of the armature winding.
Recently, a vehicle AC generator that has a pair of armature windings and a pair of rectifiers, as shown in FIG. 12, has come into wide use in order to reduce noises. If one of the pair of rectifiers of a vehicle AC generator that connects to one of the pair of armature winding fails, the duty ratio of the output voltage of the other armature winding does not change. For example, if any of diodes forming one of a pair of rectifiers for supplying the rectified output voltage of one of a pair of armature windings to a voltage regulator that has abnormality detecting function, it is possible to detect such abnormality by the control apparatus disclosed in JP-A-8-65914. However, the output voltage of the other armature winding does not significantly change even if one of the diodes of the other rectifier fails. Thus, it is not possible to detect abnormality of the other rectifier that is not connected to the voltage regulator.
In order to detect abnormality of both the rectifiers, it is necessary to detect the output voltages of both the armature windings. However, the number of input terminals of the control apparatus increases and the structure becomes complicated, resulting in low reliability of the circuit.
Although the failure warning indicator disclosed in U.S. Pat. No. 4,242,674 has a function of preventing the above problem, it has the following problems.
In a generator that is always driven at a constant rotation speed, such as a generator for an airplane, the failure warning indicator disclosed in U.S. Pat. No. 4,242,674 is very useful. However, the rotation speed of the vehicle AC generator that is driven by a vehicle engine always changes, and the amplitude of the ripple of the output voltage thereof always changes. In addition, the electric load of the AC generator changes frequently because a brake lamp frequently turns on or off when the vehicle decelerates or stops at a traffic signal. The detection accuracy may lowers if maintenance service is provided at comparatively longer intervals relative to the lifetime thereof. Therefore, it is very difficult to detect abnormality of the vehicle AC generator only by the ripple of the output voltage thereof.
In order to prevent the above inconvenience, U.S. Pat. No. 4,315,204 discloses a vehicle AC generator that employs a ripple detection means. However, it is not possible to accurately detect abnormality only by detecting the amplitude of the ripple. Therefore, the disclosed vehicle AC generator includes various other detecting means, which causes the structure complicated, large and impractical.
U.S. Pat. No. 4,178,546 discloses a device that detects frequency of ripples of DC output voltage of a generator and judges abnormality if the frequency is different from a predetermined value. However, it is very difficult to detect the frequency of the ripples that change with the rotation speed of the generator because the generator is driven by an engine whose rotation speed always changes. For example, the device disclosed in U.S. Pat. No. 4,178,546 needs a tracking filter, which is too large to be mounted in a voltage regulator of a generator.
There is an idea that the output current of a generator is directly detected. However, it is necessary to employ an expensive sensor of high response and accuracy.